Optimizing Straw Return to Enhance Grain Production and Approach Carbon Neutrality in the Intensive Cropping Systems

Straw return into agricultural soil is beneficial to agricultural production and has been widely recommended as a practice to enhance both productivity and soil fertility. However, long-term excessive straw return may be detrimental in intensive and high-yielding cropping systems. Here, we conducted a 3-year field experiment in a wheat-maize (Triticum aestivum and Zea mays) double cropping system to investigate the impacts of various straw return rates on crop productivity and carbon footprint. The soil type of the experimental site is Hapludalf. Our results revealed that during the study period from 2014 to 2017 returning 50 % of the straw from both crops (about 3.8 t C ha-1 input) led to maximum increase in grain yield by 15 % and the maximum efficiency of soil to sequestrate 24 % of carbon contained in returned straw. Returning only 25 % of straw (2.0 t C ha-1 input) maintained the relative balance of soil carbon. 75 % straw return (5.4 t C & sdot;ha-1 straw carbon) resulted in the maximum soil carbon sequestration of 0.8 t C ha-1 yr-1 and minimum carbon footprint of 2.4 t CO2-eq ha-1, but more straw return did not produce significant positive benefits. Straw return promoted farmland CO2 emission, which was equivalent to 43 % of the straw carbon input. Each 25 % increase of straw return amount increased the total direct N2O emissions by 0.5 kg N2O ha-1. Our results clearly indicate that the currently and widely practiced straw management i.e. returning all wheat and maize straw, leads to excessive carbon return, causing imbalance of soil carbon and nutrient and reduced crop yield, is therefore not the best options. Returning 50-75 % of crop straw and using the rest as stock feed, will boost crop productivity while maintaining lower carbon footprint. Our approach provides a practical and reliable method to develop a "win-win" strategy for straw management in the double-cropping systems. The optimal straw management will change with time due to changed climate, soil and management conditions,while the approach can be applied to investigate optimal straw management in all systems across environments. Although our study is constrained to short-term observations, the findings provide valuable guidance for the development of mutually beneficial crop straw management strategies and establish a solid foundation for future long-term research in this area.